Tryptophan hydroxylase appears to be an oxygen sensitive, iron requiring enzyme. Exposure of the crude enzyme to 100% O2 for 30 min. at 37 degrees C produces a 70% reduction in activity. Superoxide and hydroxyl radicals are not involved in the inactivation. There is no loss of activity at higher temperatures if the enzyme is kept under N2. Activity can be recovered by the addition of thiols. Iron chelators can also strongly inhibit tryptophan hydroxylase, at least in part, by competitive inhibition for the substrates 6MPH4 and tryptophan. If the enzyme is exposed to 25 degrees C in room air (20% O2) for 24-48 4 hrs, a substantial loss of activity is observed. Activity can be largely recovered by incubating the enzyme (24 hrs) under anerobic conditions in the presence of dithiethreitol and iron at 25 degrees C. Preliminary experiments have also suggested that the addition of inorganic sulfide produces an additional recovery of activity. Finally, sulfhydryl reagents strongly inhibit tryptophan hydroxylase. Taken together, these data suggest that tryptophan hydroxylase can exist in various activity states depending on the extent of oxidation or reduction of critical SH groups. The enzyme may be an ironsulfur protein.